You've gotten some good advice already. Another consideration may be the fin blade stiffness. Matching the diver's strength and technique with the proper stiffness, will often improve performance.
The question was raised about what kinds of exercises can be done on dry land. One exercise which may help with "ankle" strength is a reversed calf raise.
You stand on a step with only the heel on the step - maybe 2/3 of the forward portion of the foot is hanging in the air. Then you lower your toes as much as possible and then raise them, bringing your toes up toward your knees. At the top of the motion, when the shin muscles are undergoing maximum contraction, you can pause for a moment and squeeze and hold and flex the muscle. It is basically like tapping your toes.
This is a somewhat strange motion to learn, you can keep your knees slightly bent, but do not let the knees move at all. You really want to try to keep your body from rocking front to back. You will need to hold onto a hand rail gently to help balance. especially as it takes some time to learn.
You may need to do 75 or 100 repetitions, before you feel any work being done, but each repetition can be done quickly in a second or two, so after a minute or two, you will begin to feel a burn. You will may want to do 3 sets or so, per session. During the "rest" phase, you can spin around and do calf raises, (the opposite motion) and strengthen those (opposing) muscles as well. The exercise itself is pretty easy, so don't jet discouraged after 40 repetitions and not feeling anything. If your rest periods are short, maybe 60 or 75 seconds, the later sets will cause you to fatigue much quicker and you should be able to feel a burn.
Another "dry" exercise you can do is to lay on your back, raise your feet about 6 inches, point your toes and then perform a scissors kick with the knees straight or nearly so. This will work the abs and the top of the thighs, which are definitely called upon when you are doing the long, nearly straight legged kick from the hips.